Articular cartilage covers the ends of all bones that form articulating joints in humans and animals. The cartilage is made of cells called fibrochondrocytes and an extracellular matrix of collagen fibers as well as a variety of proteoglycans. The cartilage acts in the joint as a mechanism for force distribution and as a lubricant in the area of contact between the bones. Without articular cartilage, stress concentration and friction would occur to the degree that the joint would not permit ease of motion. Loss of the articular cartilage usually leads to painful arthritis and decreased joint motion.
It is known that fibrochondrocytes, the cells that produce articular cartilage, have the ability to migrate into a defect filled with a fibrin clot and form tissue substantially similar to natural cartilage. Additionally, it has been shown that fibrochondrocytes in tissue culture are capable of cell division and matrix synthesis (Webber et al. (1985) J Ortho. Res. 3(l):36). However, the amount of cartilage formed by these procedures is generally not adequate to replace severely damaged joint surfaces in vivo.
Since joint cartilage in adults does not naturally regenerate to a significant degree once it is destroyed, damaged adult articular cartilage has historically been treated by a variety of surgical interventions including repair, replacement, or by excision. With repair or excision, regeneration of tissue may occur, although the tissue is usually temporary and inadequate to withstand the normal joint forces.
Replacement of articular cartilage usually has been by allografting (Sengupta et al. (1974) J Bone Suro. 56B(1): 167-177; Rodrigo et al. (1978) Clin Orthoo. 134:342-349) by periosteal grafts (see, e.g., Engkvist (1979) Scan J Plast. Reconstr. Suro. 13:361-369; Rubak (1982) Acta Orthop. Scan. 53:181-186) or with metal and/or plastic components (Rubash et al., eds. (1991) Clin. Orth. Rel. Res. 271:2-96). Allografting dead cartilage tissue has been tried for years with minimal success. This approach has been only partially successful over the long term due to the host's immunologic response to the graft, failures in the cryopreservation process, and failures of the attachment sites. Replacement of an entire joint surface with metal and plastic components has met excellent success for the older, more sedentary patients, but is generally considered insufficient for tolerating the impact of athletic activities, and has not been shown to restore normal joint mechanics.
In alternative prior art approaches, articular cartilage has been replaced with prostheses composed of bone and/or artificial materials. For example, U.S. Pat. No. 4,627,853 describes the use of demineralized allogenic or xenogenic bone segments as replacements. The proper functioning of these replacements depends on the differential demineralization of the bone segments. U.S. Pat. No. 4,846,835 describes a grafting technique for transplantation of fibrochondrocytes to promote healing lesions in articular cartilage. U.S. Pat. No. 4,642,120 describes the use of gel-like compositions containing embryonal fibrochondrocytes. U.S. Pat. No. 5,306,311 describes a prosthetic articular cartilage which includes a dry, porous volume matrix adapted to have in vivo an outer contour substantially the same as that of natural articular cartilage.
Despite these developments, the replacement of cartilage tissue with structures consisting of permanent artificial materials generally has been less than satisfactory, and a structure suitable as articular cartilage and constructed from natural resorbable materials, or analogs thereof, has not been developed. Because the opposing articular cartilage of mammalian joints is so fragile, it will not withstand abrasive interfaces nor compliance variances from normal which eventually result from the implantation of prior art artificial cartilage. Additionally, joint forces are multiples of body weight which, in the case of the knee and hip, are typically encountered over a million cycles per year. Thus far, prior art permanent artificial cartilages have not been composed of materials having natural articular cartilage properties, nor have they been able to be positioned securely enough to withstand such routine forces.
Much of the structure and many of the properties of original tissues may be retained in transplants through use of xenogeneic or xenograft materials, that is, tissue from a different species than the graft recipient. For example, tendons or ligaments from cows or other animals are covered with a synthetic mesh and transplanted into a heterologous host in U.S. Pat. No. 4,400,833. Flat tissues such as pig pericardia are also disclosed as being suitable for heterologous transplantation in U.S. Pat. No. 4,400,833. Bovine peritoneum fabricated into a biomaterial suitable for prosthetic heart valves, vascular grafts, burn and other wound dressings is disclosed in U.S. Pat. No. 4,755,593. Bovine, ovine, or porcine blood vessel xenografts are disclosed in WO 84/03036. However, none of these disclosures describe the use of a xenograft for articular cartilage replacement.
Once implanted in an individual, a xenograft provokes immunogenic reactions such as chronic and hyperacute rejection of the xenograft. The term "chronic rejection", as used herein refers to an immunological reaction in an individual against a xenograft being implanted into the individual. Typically, chronic rejection is mediated by the interaction of IgG natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells, and/or cellular and/or extracellular matrices of the xenograft. For example, transplantation of cartilage xenografts from nonprimate mammals (e.g., porcine or bovine origin) into humans is primarily prevented by the interaction between the IgG natural anti-Gal antibody present in the serum of humans with the carbohydrate structure Gal.alpha.1-3Gal.beta.1-4GlcNAc-R (.alpha.-galactosyl or .alpha.-gal epitope) expressed in the xenograft. K. R. Stone et al., Porcine and bovine cartilage transplants in cynomolgus monkey: I. A model for chronic xenograft rejection, 63 Transplantation 640-645 (1997); U. Galili et al., Porcine and bovine cartilage transplants in cynomolgus monkey: II. Changes in anti-Gal response during chronic rejection, 63 Transplantation 646-651 (1997). In chronic rejection, the immune system typically responds within one to two weeks of implantation of the xenograft.
In contrast with "chronic rejection", "hyper acute rejection" as used herein, refers to the immunological reaction in an individual against a xenograft being implanted into the individual, where the rejection is typically mediated by the interaction of IgM natural antibodies in the serum of the individual receiving the xenograft and carbohydrate moieties expressed on cells. This interaction activates the complement system causing lysis of the vascular bed and stoppage of blood flow in the receiving individual within minutes to two to three hours.
The term "extracellular matrix or matrices", as used herein, refer to collagen and elastic fibers as well a variety of proteoglycans, which are secreted by fibrochondrocytes during cartilage growth and which undergo slow turn-over.
Xenograft materials may be chemically treated to reduce immunogenicity prior to implantation into a recipient. For example, glutaraldehyde is used to cross-link or "tan" xenograft tissue in order to reduce its antigenicity, as described in detail in U.S. Pat. No. 4,755,593. Other agents such as aliphatic and aromatic diamine compounds may provide additional crosslinking through the side chain carboxyl groups of aspartic and glutamic acid residues of the collagen polypeptide. Glutaraldehyde and diamine tanning also increases the stability of the xenograft tissue.
Xenograft tissues may also be subjected to various physical treatments in preparation for implantation. For example, U.S. Pat. No. 4,755,593 discloses subjecting xenograft tissue to mechanical strain by stretching to produce a thinner and stiffer biomaterial for grafting. Tissue for allograft transplantation is commonly cryopreserved to optimize cell viability during storage, as disclosed, for example, in U.S. Pat. Nos. 5,071,741; 5,131,850; 5,160,313; and 5,171,660. 5,071,741 discloses that freezing tissues causes mechanical injuries to cells therein because of extracellular or intracellular ice crystal formation and osmotic dehydration.
A need for an improved replacement for articular cartilage which is biocompatible, soft, lubricating, and durable continues to exist.